TOPAS-Academic is a Windows based general non-linear least squares system driven by a scripting language. Its main focus is in crystallography, solid state chemistry and optimization. At the core of TA is its superb minimization routines wrapped by a computer algebra system; it is this foundation that makes subject dependent modules such as Rietveld refinement shine. The main features include: Computer algebra system for function minimization and for the application of linear/non-linear constraints. A fully functional Rietveld program for laboratory x-ray diffraction, synchrotron, single crystal and neutron fixed wavelength and TOF data. A simulated annealing algorithm for all systems including structure solution in real space from powder, neutron, neutron TOF and single crystal data. An indexing algorithm. A command line program called TC.EXE which comprises the Kernel of TA; used for batch processing